autofs: use flexible array in ioctl structure

Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") introduced a warning
for the autofs_dev_ioctl structure:

In function 'check_name',
inlined from 'validate_dev_ioctl' at fs/autofs/dev-ioctl.c:131:9,
inlined from '_autofs_dev_ioctl' at fs/autofs/dev-ioctl.c:624:8:
fs/autofs/dev-ioctl.c:33:14: error: 'strchr' reading 1 or more bytes from a region of size 0 [-Werror=stringop-overread]
33 | if (!strchr(name, '/'))
| ^~~~~~~~~~~~~~~~~
In file included from include/linux/auto_dev-ioctl.h:10,
from fs/autofs/autofs_i.h:10,
from fs/autofs/dev-ioctl.c:14:
include/uapi/linux/auto_dev-ioctl.h: In function '_autofs_dev_ioctl':
include/uapi/linux/auto_dev-ioctl.h:112:14: note: source object 'path' of size 0
112 | char path[0];
| ^~~~

This is easily fixed by changing the gnu 0-length array into a c99
flexible array. Since this is a uapi structure, we have to be careful
about possible regressions but this one should be fine as they are
equivalent here. While it would break building with ancient gcc versions
that predate c99, it helps building with --std=c99 and -Wpedantic builds
in user space, as well as non-gnu compilers. This means we probably
also want it fixed in stable kernels.

Cc: stable@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230523081944.581710-1-arnd@kernel.org

docs: filesystems: Fix grammatical error "with" to "which"

Signed-off-by: Wasin Thonkaew <wasin@wasin.io>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: Add mount map description in Content

The second paragraph of the content section does not properly
describe how mount points are determined by autofs.

Replace the lines detailing how the determination of these mount
points is "ad hoc" by a short description of the mount map syntax
used by autofs.

Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: Update code snippets in autofs.rst

Some of the struct definitions now have an autofs packet header.
Reflect these changes by adding a definition of this header and
place it wherever suitable.

Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: convert autofs.txt to reST

Convert autofs.txt to reST.

The following changes abound:

- Introduce reST formatting for headings, lists et al.
- Add an indentation of an 8 space tab wherever suitable, so as
to maintain consistency.
- Remove indentation of the description of the ioctls which are similar
to the AUTOFS_IOC ioctls, as it does not come out quite right in HTML.
- Add an entry for autofs in the index.

Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>